1. Field of the Invention
The present invention relates to a method for controlling an exhaust gas purification system having a diesel particulate filter device and a selective catalytic reduction type catalytic device provided, in that order, from the upstream side of the exhaust passage of an internal combustion engine such as a diesel engine.
2. Description of the Related Art
In order to reduce particulate matter (PM) and NOx (nitrogen oxides) exhausted from internal combustion engines such as diesel engines, some exhaust gas purification systems are provided with a diesel particulate filter device (DPF device) and a NOx reduction catalytic device (SCR catalytic device) that supports a selective catalytic reduction type catalyst (referred to as an SCR catalyst).
As an example of such exhaust gas purification systems, exhaust gas purification devices for diesel engines have been proposed as described in Japanese Patent Application Kokai Publication No. 2002-250220 (Patent Document 1), for example, wherein, with the aim of improving the NOx removal rate as well as preventing ammonia slip, a NOx catalyst (SCR catalytic device) is disposed posterior to a particulate matter reducing device (DPF device) in an exhaust system, the ratio NO/NO2 is calculated based on the temperature at the outlet of the particulate matter reducing device, and an amount of urea supplied to the NOx catalyst is controlled based on this calculated value.
In addition, exhaust gas treatment systems have been proposed as described in Japanese Patent Application Kokai Publication No. 2004-218475 (Patent Document 2), for example, wherein a DPF device, a selective catalytic reduction type catalyst (SCR catalytic device), and a NOx storage reduction catalyst are provided from the upstream side of the exhaust passage of an internal combustion engine. The exhaust gas is purified by the removal of NOx thereof by the selective catalytic reduction type catalyst at high temperatures and by the NOx storage reduction catalyst at low temperatures, and thus the system removes NOx from exhaust gas over a wide range of engine operation and at a high NOx removal rate.
In addition, exhaust gas treatment devices have been proposed as described in Japanese Patent Application Kokai Publication No. 2004-353523, for example, wherein favorable NOx removal effects are obtained for the entire operational range of the engine by circulating and mixing ammonia with exhaust gas, the ammonia being formed by vaporizing aqueous urea solution using the heat of the exhaust gas and the heat from a heater. The mixture is then introduced into a denitration catalytic reactor and NOx is removed.
In these devices and systems, the supplied amount of aqueous urea solution is adjusted and controlled based on factors such as: the NOx emission amount obtained from a NOx emission map that is based on the engine speed and torque; NOx concentration measured by a NOx sensor at the downstream side of the DPF device and the NOx amount present in exhaust gas calculated from the exhaust amount; and all or part of the output of an exhaust gas temperature, exhaust gas flow amount, and nitrogen oxides sensor.
However, in this exhaust gas purification system provided with a DPF device and a SCR catalytic device, it is necessary to conduct a forced DPF regeneration control to remove deposited PM by combustion for clogging of the DPF device due to PM deposits. In this forced DPF regeneration control it is typical to use techniques such as halting EGR or releasing air intake throttling in order to promote PM combustion.
During ordinary operation, NOx emission is suppressed by EGR. However, in the forced DPF regeneration control, EGR is halted in order to increase oxygen concentration and promote combustion of PM (soot) deposited on the DPF. Halting EGR promotes the formation of NO2 (nitrogen dioxide) to promote reactions between NO2 and PM, which more readily react in a low-temperature zone compared to O2 (oxygen) and PM, and thus the promoted reactions provide the effect of efficiently conducting DPF regeneration and reducing the PM originating from the engine.
In this forced DPF regeneration control, the engine operating state becomes different from the ordinary, and a large amount of NOx is exhausted, this amount being different from the NOx emission amount calculated from the NOx emission map set in advance for ordinary operation. For example, bench test results of actual engines showed that the NOx emission amount during forced DPF regeneration was measured nearly double that of ordinary operation.
Consequently, there is a problem in that during the forced DPF regeneration control, in the case where an injection amount (supply amount) of aqueous urea solution is calculated and injected on the basis of NOx emission amounts estimated for the ordinary operational state, there is the possibility that the system will not be able to sufficiently reduce NOx exhausted from the engine, the NOx removal rate during forced DPF regeneration will greatly decrease, and NOx exceeding the emission level during ordinary operation will be exhausted to the downstream side of the SCR catalytic device.
Moreover, in the opposite case where an injection amount of aqueous urea solution is uniformly calculated and injected on the basis of a map based on NOx weights estimated for the forced DPF regeneration, there is the problem in that during ordinary operation, there arises the possibility that unconsumed surplus ammonia will be exhausted to the downstream side of the SCR catalytic device, and thus the amount of ammonia slip will increase.    Patent Document 1: Japanese Patent Application Kokai Publication No. 2002-250220    Patent Document 2: Japanese patent application Kokai publication No. 2004-218475    Patent Document 3: Japanese patent application Kokai publication No. 2004-353523